This invention relates to a centrifugal separator for separating solid particles or colloidal particles suspended in fluid by subjecting them to the centrifugal force, and more particularly to a centrifugal separator for separating solid particles from a gas and used in a multicentrifuge.
As a means to remove particles suspended in gases a multicentrifuge has been used widely which containes a number of relatively small unit centrifugal separators in it. The multicentrifuge has a problem that there is a large difference in performance between the small unit centrifugal separators contained in the multicentrifuge and the multicentrifuge as a whole. For example, each unit centrifugal separator can remove 100% of the suspended particles of a size more than 10.mu., while the multicentrifuge can remove only 20 to 30% of them.
According to the conventional explanation, the big difference in performance between the unit centrifugal separator and the multicentrifuge was said to be attributable to the ununiform velocity of gas at the inlet of the centrifugation chamber of the multicentrifuge. This is not logical, however, considering the fact that the speed of gas flow immediately before and within the centrifugation chamber is about 5 to 10 times the speed of gas being led to the multicentrifuge. Further, it has been found with experiment that the gas flow speed at the entrance of the centrifugation chamber is almost uniform and cannot cause such a drastic deterioration of the performance. A conventional centrifugal separator is provided with a centrifugation chamber formed cylindrically and having relatively narrow particle discharge port at the lower end, an inlet body disposed at the upper portion, and a pipe for discharging clean gas separated of particles. A gas containing particles is introduced into the centrifugation chamber from the inlet body so as to whirl so that the particles in the gas are forced radially outwardly by the centrifugal force exerted upon them. The particles gradually move down circling along the cylindrical inner wall of the centrifugation chamber until they reach the particle discharge port, then they are exhausted into a particle accumulator chamber below the centrifugation chamber. The clean gas removed of the particles is led up through the opening of the gas discharge pipe disposed at the center of the centrifugation chamber into a clean gas chamber out of the centrifugation chamber. This is a general explanation on the principle of the conventional centrifuge. If a plurality of unit centrifuges are put together and used as a multicentrifuge, the performance decreases greatly as mentioned previously. The real cause of this performance deterioration has so far not been throughly investigated. Our research and experiments have shown that this is caused by the powerful circulating motion of the gas at the particle discharge port of the centrifugation chamber. The strong circling motion of the gas and particles at the particle discharge port causes the gas and particles in the particle accumulator chamber to be drawn up into the central portion of the centrifugation chamber. In other words, the outgoing and incoming gas flows passing through the particle discharge port deteriorate the performance of the multicentrifuge. The unit centrifugal separator also has the particle flow and the gas flows passing through the particle discharge port. But with the unit centrifugal separator, the circling motion of the gas is maintained in the particle accumulator chamber disposed outside the particle discharge port, so that the particles in the accumulator chamber is forced radially outwardly or away from the center. As a result, the gas flow returning into the centrifugation chamber contains almost no particles. On the other hand, in the multicentrifuge which has a number of closely arranged particle discharge ports, the whirling return gas flows disturb each other producing complex turbulent flows in the particle accumulator chamber. This causes the particles to be drawn into the centrifugation chamber, together with the return gas flow. This particle flow is then led through the pipe into the clean gas chamber greatly reducing the performance of the multicentrifuge. The performance of the multicentrifuge is determined by the speed of the turbulent flow in the particle accumulator chamber and the speed at which particles sink by gravity. For example, for the particles with specific gravity of 2 to 3, the natural sinking speed is 30 cm/sec for the particle size of 100.mu. and 0.3 cm/sec for the particle size of 10.mu.. Considering the turbulent flow speed of 100 cm/sec in the particle accumulator chamber, it can safely be concluded that satisfactory separation cannot be expected even for the large particle size of 100.mu. and that for the small particle size of about 10.mu. no separation will be achieved.
As mentioned above, the conventional multicentrifuge has very low performance for the particle size less than 100.mu. because of the strong turbulent return flows which send particles up into the centrifugation chamber and into the clean gas chamber. The other prior arts are as follows:
1. Japanese patent publication No. 43-22818 (1968) "Centrifuge"
This is concerned with the centrifuge having a closing member at the lower portion of a centrifugation chamber. The closing member is provided with an annular groove and holes at the circumference.
2. U.S. Pat. No. 3,074,218 (1968) "Gas cleaner"
The gas cleaner disclosed by FIG. 3 has a disk with a hole at the lower portion of a centrifugation chamber. Particles separated from gas are discharged from a hole made in the side wall of the chamber.
3. U.S. Pat. No. 2,981,369 (1961) "Vortical whirl separator"
This is concerned with vortical whirl separator having a closing member with holes.